The Capitol Grounds a.k.a. Capitol Park (I), was a baseball field in Washington, D.C. The grounds were the home field for the Washington Nationals of the Union Association during the league's only season in 1884. The ballpark had a seating capacity of 6,000, and was where the Russell Senate Office Building stands today.

The ball field was bounded by C Street NE (north); Delaware Avenue NE (west): B Street (now Constitution Avenue) NE (south); and First Street NE (east); just northeast of the Capitol building.

After dropping out of the Union Association, the club joined the minor Eastern League for the 1885 season. The club regrouped as a new entry to the National League in 1886, and moved a few blocks north to the larger Capitol Park (II).

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